Because of recent epizootics of mousepox at NIH and other biomedical research institutions in the U.S., research was undertaken to develop an in vitro assay for infectious virus. In addition, a study of biological and biochemical markers of virus strains in varous strains of inbred mice as correlates of epizootiology and pathogenesis of mousepox are in progress. This basic biological information and diagnostic methodology are considered essential for a rational approach to the control and prevention of mousepox, which threatens serious disruption of biomedical research in the U. S.